1. Field of the Invention
The present invention relates to an optical scanning apparatus with a semiconductor laser, and a color image forming apparatus that is provided with this optical scanning apparatus like a copying machine or a printer with an electrophotography system.
2. Description of the Related Art
An optical scanning apparatus that is mounted in an image forming apparatus for printing an image using an electrophotography process is generally configured to reflect a laser beam emitted from a light emission unit by a rotating polygon mirror, and to form a linear electrostatic latent image by scanning a photoconductive drum with a spot formed on the photoconductive drum through an fθ lens.
Such an optical scanning apparatus is provided with a BD sensor that detects light receiving timing of the laser beam at a position outside a image forming area in order to acquire a synchronized signal for determining a rendering start point. In order to perform light control (Auto Power Control: APC) so as to keep a density of an image in a predetermined level, an APC sensor that detects the light amount of the laser beam is required.
The BD sensor preferably detects the laser beam that scans in the same speed as the beam scans on the photoconductive drum. On the other hand, the APC sensor preferably detects the laser beam that scans in lower speed than the scanning speed on the photoconductive drum in order to detect light amount correctly, and preferably uses an optical system of which focal length is shorter than that of the optical system for rendering. Japanese Laid-Open Patent Publication (Kokai) No. H9-146025 (JP H9-146025A) discloses a configuration that uses one sensor as both the BD sensor and the APC sensor. However, the configuration disclosed in this publication cannot detect the laser amount of each laser beam correctly, particularly when a plurality of laser beams are used, because the scanning speed of the laser beam that runs across the sensor is too high. That is, the BD sensor and the APC sensor should be provided independently in order to detect light amount correctly.
In an image forming apparatus, variations of a position of each optical element and refractive index of each lens due to heat produced by various heat sources, such as motors, a fixing heater, and a power source, deviate a converging position of the laser beam from the photosensitive drum, which enlarges the diameter of spot formed on the photoconductive drum.
Especially, since a high-definition optical scanning apparatus of which the spot diameter is small becomes shallow in focal depth at the side of the photoconductive drum, the spot diameter remarkably expands due to the influence of heat. Such an apparatus needs to detect change (defocus amount) of the converging position of laser beam with an autofocus (AF) sensor and to correct the change with an AF mechanism.
Japanese Laid-Open Patent Publication (Kokai) No. 2008-122613 (JP 2008-122613A) discloses a configuration that uses one sensor as both the BD sensor and the AF sensor. The focus detection method disclosed in this publication moves a collimator lens in an optical axis direction so as to maximize the peak of differential value of the sensor output (light amount) using characteristics that the peak of differential value of the sensor output at the time when the laser beam runs across the sensor increases as the spot size decreases.
However, since the technique of JP 2008-122613A only detects the defocus amount in the principal scanning direction, it is insufficient for applying to an anamorphic optical system in which powers are different in a principal scanning direction and an auxiliary scanning direction.
On the other hand, Japanese Laid-Open Patent Publication (Kokai) No. 2010-096898 (JP 2010-096898A) discloses an optical scanning apparatus provided with an AF mechanism that is suitable for an anamorphic optical system. The apparatus disclosed in this publication is provided with a separator lens and an AF sensor. The separator lens has four lens parts for dividing a laser beam passed through an fθ lens into four spots (two spots divided in the principal scanning direction and two spots divided in the auxiliary scanning direction). The AF sensor detects a gap between the two spots divided in the principal scanning direction as defocus amount in the principal scanning direction, and detects a gap between the two spots divided in the auxiliary scanning direction as defocus amount in the auxiliary scanning direction. The apparatus moves a collimator lens and a cylindrical lens in the optical axis direction based on the defocus amounts in the principal scanning direction and the auxiliary scanning direction.
However, since the AF sensor is not suitable for detecting the light amount of the laser beam and the light receiving timing in the configuration that detects the defocus amount by dividing the laser beam as disclosed in JP 2010-096898A, the AF sensor cannot be used as both suitable for detecting the light amount and the light receiving timing of a laser beam, and the AF sensor cannot be used as a BD sensor or an APC sensor.
Accordingly, the optical scanning apparatus using a plurality of laser beams and an anamorphic optical system needs exclusive sensors for the synchronous control, the light control, and the focus control, respectively. However, when three kinds of sensors are arranged side by side outside the image forming area, the emission time for the APC sensor, the emission time for the BD sensor, and the emission time for the AF sensor are added to the emission time for forming an image. Accordingly, the emission time of the light emission unit with a semiconductor laser increases, which causes a problem of shortening the use life of the semiconductor laser.